What do you mean by top-up and credit here?
Cash credit/top-up?
Membership?
Add-ons?
What exactly are you trying to buy?
What does your My 48 account page say about your current membership status and expiry date and does it list any "next" membership(s) in the membership planner section?
https://www.48months.ie/my-48What does your My history page say about recent payments/purchases?
http://www.48months.ie/my-48/my-historyAnswering all of these questions should clarify matters.
Hope this helps.